Soldering involves the physical and electrical connection of components or devices using a low melting point alloy. Examples of suitable alloys include tin-lead (approximately 60% -40% by weight) solder having a melting point of approximately 178 degrees Celsius (.degree.C.) and indium-lead (70% -30% by weight) solder having a liquidus temperature of approximately 174.degree. C., as well as many others. Such solders may be used to solder particular materials, for example, the indium-lead solder mentioned above is useful for soldering gold or gold plated components which are chemically and/or metallurgically attacked by tin-lead solders. Such gold or gold plated components may comprise gold bearing alloys such as, by way of example, eutectic alloys of gold-tin, gold-germanium, gold-silicon and other gold bearing materials as are well known in the art.
Soldering processes involve the basic steps of cleaning and de-oxidizing, solder reflowing and flux residue removal. Cleaning and de-oxidizing are usually accomplished by applying a flux material to remove contaminants and oxides from the surfaces to be soldered. Oxides typically have higher melting points than solder alloys and may form an insoluble barrier, preventing wetting of the surfaces to be soldered if the oxides are not removed prior to the solder reflowing. Solder reflowing joins the surfaces to be soldered when the solder is heated above its melting point. Residue removal involves stripping of flux residues left from the cleaning and de-oxidizing step and this becomes more difficult as the physical size of components to be soldered decreases. This is because it is more difficult for the residue cleaning agents to penetrate small gaps between the components and the substrate.
Hand soldering involves soldering each solder joint by hand, one at a time. This method also requires use of flux during soldering and subsequent flux removal after soldering. In hand soldering, components are subjected to localized heating which may affect the material in the component or substrate, depending on the time and temperature required in order to make the solder joint. The substrate and component are each subjected to a high temperature in a localized area. Because of the thermal mass of the component or area being soldered, the material in the area being soldered generally must be heated 20.degree.-40.degree. C. above the solder melting temperature, increasing the potential for damage. Hand soldering is labor intensive inasmuch as each soldered connection must be made sequentially. These limitations have led to intense development of alternative methods wherein multiple solder connections are effected by a single operation.
Examples of such methods include both wave soldering and vapor phase soldering. These techniques may be used to heat the solder (and the substrate on which the components are to be mounted) to the melting or the liquidus temperature of the solder. Both methods heat all components and the substrate to the soldering temperature and also require use of a flux. Thus, all components must be capable of withstanding the soldering temperatures and cannot be affected by either the flux or the cleaning solutions utilized to remove flux residues.
The cleaning solutions employed for flux removal pose environmental hazards on use and on disposal, may pose health hazards for workers, and may necessitate expensive apparatus for their use and for exhaust and effluent scrubbing. A further issue which use of some solvent cleaning methods raise is that of minimizing the risk of fire due to presence of flammable solvent vapors.
Another disadvantage of soldering methods wherein fluxes are included is that complete removal of residues from the flux is not possible. As a result of this, standards for specifying post-soldering flux criteria are directed to determining residual concentrations of flux which are acceptable for a given application.
Several fluxless soldering processes have been developed to replace the pre-soldering cleaning step and to eliminate need for post-soldering flux residue removal and the attendant need for and risks associated with solvent use. Among these processes are sputtering, fluorinated gas plasma use and use of oxygen and pure nitrogen plasmas. Sputtering is limited in accuracy and penetrates for only short distances. Also, sputtering may damage the substrates and components.
Fluorinated gas plasmas attack certain materials (e.g., glasses) and require exhaust scrubbing systems in order to meet environmental regulations. Oxygen plasmas are useable with gold eutectic solder alloys, but badly oxidize tin-lead solder. Pure nitrogen plasmas do not generally provide sufficient fluxing action to allow tin-lead solder to wet either the substrate or the components to be soldered. Also, solder reflow in fluorinated gas, oxygen or nitrogen plasmas is typically accomplished by conventional heat application from a heat source other than from the plasma itself (e.g., infrared heat sources, laser heating, et cetera).
An additional series of problems are encountered in attempting to solder a two-dimensional array of solder connections captive between two planar surfaces which are in close proximity to one another. There is difficulty in removing flux from between two planar, closely spaced surfaces. A further difficulty occurs in inspecting for complete flux removal after soldering without destructive physical analysis.
What is needed is a method for fluxless soldering obviating post-soldering flux removal, which does not require direct contact between the plasma and the surfaces to be solder joined, allowing solder connections to be formed which are shielded from the plasma. What is further needed is a method for fluxless soldering which does not pose environmental concerns, which allows a plurality of solder joints to be simultaneously established and which will not attack the components being soldered.